This application is related to the drilling of underground formations containing highly abrasive materials and is more particularly directed to methods and apparatus for drilling through abrasive formations utilizing high drag and intermittent contact techniques.
There are many known drilling tools utilized in the prior art for penetrating underground formations with drilled bore holes. Many of these prior art devices utilize a tricone rolling cutter drill bit having three conical cutter heads rotatably mounted on journal bearing shafts. Another type of drilling device commonly utilized in this field is the diamond type of bit which utilizes the single drilling head formed of a single integrated body which drilling head contains cutting elements and is rotated against the formation being cut. The diamond type bit normally maintains full drilling contact with the formation at all times during the drilling operation. The rolling cutter drill bit normally maintains full or nearly full surface contact between the three rolling cutter lower edges and the bottom of the bore hole. Normally the tricone bit utilizes very little drag and in many instances is defined as a "rolling cone" bit.
The problems arising from these two types of drilling bits depend upon the principle involved in the drilling operation. In a highly abrasive formation the diamond type bit which utilizes either natural or synthetic diamond cutting elements located in the single integral head structure is that the drilling head and the cutting elements maintain constant contact with the abrasive formation at all times during the drilling operation. Because of the highly abrasive nature of certain formations, a large amount of heat buildup occurs in the cutting elements. Because of the high conductivity of the diamond portion of the cutting element and the lower conductivity of the carbide mounting in the element and also the low conductivity of the carbide-diamond interface, the heat generated by drilling in the abrasive formations is trapped in the diamond cutting element and serves to rapidly deteriorate the element. Because of this heat buildup the life of diamond cutting elements in these type formations is severely restricted. In addition, the constant contact of the cutting elements causes formation material to build up at the diamond-formation interface, impeding the cutting action of the cutting elements. Attempts to alleviate these problems have been directed toward providing extremely high velocity drilling fluid past the cutting elements to cool and clean the elements. This requires a high hydraulic input of a drilling fluid into the drilling area which requires a great amount of horsepower and which can result in erosion of the exposed portions of the drilling system.
Although the rolling cutter drill bit utilizes intermittent contact drilling elements it suffers in many tough formations in that the rate of penetration is drastically reduced. To obtain an acceptable rate of penetration, the drilling bit necessarily requires a relatively high amount of drag between the cutting elements and the formation being drilled. A rolling cutter drill bit, rather than shearing the material from the rock face, generally performs the function of rock removal by compressive forces. This, while sufficient in brittle formations, does not give a good rate of penetration in extremely tough formations. Thus the rolling cutter drill bit normally does not provide sufficient drag to give an acceptable rate of penetration in the tough formations.
The present invention overcomes the disadvantages and deficiencies of the prior art devices by providing methods and apparatus for drilling abrasive formations utilizing high drag intermittent contact drilling bits.